Based on your description you have a bad throttle potentiometer or speed potentiometer. Remove the covers of the front console and check the resistance throughout the range of both pots at the connector. It should be even throughout the range with no dead spots. The newer CTE throttle pot is more reliable than the older model Celebrity scooter with the 5K ohm potentiometer. I usually replace the throttle and speed pots as a pair since they are connected electrically in the console.
Load test the batteries while you are driving. Using your voltmeter that is still connected under the rear cover while you are driving, check to see if you get a voltage drop of more than 1 volt. Static testing of batteries only tells you that there is a full charge. It does not give you an indication of how the batteries perform under load.
If you have clean brushes, you can still have a large amount of conductive brush dust in the motor and armature. Blow out the dust with compressed air. Check the motor and brake with you meter. The motor coil resistance should be less than 1.5 ohms and the brake should read between 40 to 60 ohms. Hope this helps.